1. Field of the Invention
The present invention relates to methods for manufacturing a multilayer ceramic substrate and to composite sheets. In particular, the present invention relates to a method for manufacturing a multilayer ceramic substrate in which a resistor pattern and/or conductor pattern is formed on the external surface of the multilayer ceramic substrate by an ink jet method, and to a composite sheet advantageously used in the manufacture of the multilayer ceramic substrate.
2. Description of the Related Art
A so-called constrained sintering process is a method for manufacturing a multilayer ceramic substrate relating to the present invention. For manufacturing a multilayer ceramic substrate by a constrained sintering process, a multilayer composite is prepared. The multilayer composite includes a plurality of ceramic green layers containing a low-temperature co-fired ceramic material and a shrinkage-retardant layer containing a sintering-resistant ceramic powder substantially not sintered under a condition for sintering the low-temperature co-fired ceramic material. The shrinkage-retardant layer is disposed on at least one main surface of an unfired multilayer ceramic substrate including the plurality of ceramic green layers.
The multilayer composite is subsequently fired under the condition for sintering the low-temperature co-fired ceramic material. Thus, a multilayer ceramic substrate is completed through sintering the ceramic green layers. The shrinkage-retardant layer is not shrunk by firing because the sintering-resistant ceramic powder contained in the shrinkage-retardant layer is not substantially sintered. Accordingly, the shrinkage-retardant layer restrains the ceramic green layers, so that the ceramic green layers shrink substantially only in the thickness direction, but are prevented from shrinking in the main surface direction. Consequently, the resulting multilayer ceramic substrate becomes difficult to deform non-uniformly, and the geometrical and dimensional precision of the multilayer ceramic substrate can be increased.
Subsequently, the shrinkage-retardant layer is removed, and thus a desired multilayer ceramic substrate is obtained.
If a resistor pattern and/or conductor pattern is provided on the external surface of the multilayer ceramic substrate, the resistor pattern and/or conductor pattern may be formed after removing the shrinkage-retardant layer. If the resistor pattern is provided, in general, a glass overcoat layer is additionally formed as a protective layer. The overcoat layer protects the resistor pattern from the influence of plating and ensures the reliability of the resistor pattern. For forming the resistor pattern and/or conductor pattern and the overcoat layer, firing steps are respectively performed. This means that the multilayer ceramic substrate undergoes repetition of firing steps, and the resulting multilayer ceramic substrate is brought into an excessively sintered state. Consequently, the mechanical strength and the electrical characteristics of the ceramic layers of the multilayer ceramic substrate may be varied, and the adhesion between the ceramic portion and the conductor portion may be reduced.
Accordingly, a process can be proposed in which the ceramic green layers and an unfired resistor pattern and overcoat layer are fired at one time in a state where the unfired resistor pattern and overcoat layer disposed on the outermost ceramic green layer of the multilayer composite are covered with a shrinkage-retardant layer. Thus, the ceramic green layer can avoid undergoing a plurality of firing steps.
In this process, however, when the shrinkage-retardant layer is removed, the shrinkage-retardant layer and the resistor pattern may be disadvantageously removed together, because the adhesion of the resistor pattern to the multilayer ceramic substrate is relatively low. Japanese Unexamined Patent Application Publication No. 2005-39164 proposes that the glass component contained in the overcoat layer is improved in softening point and composition to overcome the above disadvantage.
As another approach, a process is proposed in which a resistor pattern and a conductor pattern are formed on the external surface of a ceramic substrate, such as the multilayer ceramic substrate, by an ink jet method. The ink jet method can not only efficiently conduct the steps of forming the resistor pattern and the conductor pattern, but also immediately cope with the alteration of the pattern.
Unfortunately, the resistor ink and conductor ink used for the ink jet method contain a solvent and have low viscosity. Such an ink is liable to spread out undesirably when it is applied onto a normal ceramic green layer.
Accordingly, the resistor pattern and the conductor pattern cannot have high line fineness.
In addition, the solvent in the resistor ink or the conductor ink may dissolve the binder contained in the ceramic green layer, thereby roughing the surfaces of the resistor pattern and the conductor pattern.